Biogenic amines play key roles in neurotransmission, metabolism, and in control of various physiological processes. Progress has been slow in determining their mechanisms of transport, storage, release and mode of action. Ring-halogenated analogs have proved to be powerful tools for the study of these mechanisms, since they simulate the geometries of the natural compounds so well. By virtue of its very small size and high electronegativity, fluorine should be a very favorable replacement for hydrogen in these analogs. Some years ago, we developed novel methods of the synthesis of such analogs and have prepared a wide variety of biogenic amines with fluorine at various ring positions. The biological properties and usefulness of these analogs have proved unexpectedly rewarding and continue to find application in a multitude of studies. Perhaps the most significant finding, to date, is that 2-fluoronorepinephrine is a pure Beta-adrenergic agonist, while the 6-fluoro isomer is a pure Alpha-adrenergic agonist. Various explanations for the role of fluorine in creating such specificity have been considered and discarded. We now propose that a critical ion-dipole interaction exists between a positively-charged site on the receptor and an electron-rich carbon atom of the benzene ring. Inductive withdrawal by fluorine would weaken this interaction and produce the observed specificities. In the course of this work, we have prepared fluorinated analogs of norepinephrine, isoproterenol, phenylephrine, tyramine, dopamine, serotonin, melatonin, DOPA, Alpha-methylDOPA, reserpine, normetanephine, and homovanillic acid.